Apparatus for forming multicolor interference coating

ABSTRACT

Several techniques may be used for forming a colored interference filter coating on a substrate such as polyester film. The interference filter has two metal reflective films, at least one of which is semi-transparent. A layer of transparent acrylate polymer dielectric between the metal layers completes the interference filter, which may be sandwiched between protective layers. The dielectric is formed by evaporating an acrylate monomer having a molecular weight in the range of from 150 to 600. Preferably the acrylate monomer has a molecular weight to acrylate group ratio in the range of from 150 to 400. The acrylate condenses on the substrate and is polymerized in situ for forming a monolithic film with a sufficient thickness to produce an interference color. In several embodiments different areas of the film have different thicknesses for producing different interference colors. The thickness of the dielectric can be controlled by the amount of monomer condensed, by either controlling the temperature of the condensation surface or controlling the amount of monomer evaporated adjacent a predetermined area of the substrate. Thickness may also be controlled by condensing a uniform layer of monomer and polymerizing the monomer to different degrees for varying the shrinkage of the film and hence the thickness of the film and color.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 08/990,708filed on Dec. 15, 1997, which is a divisional of application Ser. No.08/406,566 filed on Mar. 20, 1995, now U.S. Pat. No. 5,877,895.

BACKGROUND

This invention relates to formation of a multicolor interference coatingfor a transparent or opaque substrate. The coating material is anacrylate polymer and different colors are obtained by having differentthicknesses of transparent coating in adjacent areas.

Interest has developed in recent years in the protection of currency andother documents from counterfeiting by use of interference filters. Thecolor variations available from interference filters cannot beduplicated by copying machines. The specialized equipment needed forproducing the interference filters is not readily available tocounterfeiters. Thus, it has been proposed to mark Canadian currencywith multicolored interference filter patterns to inhibitcounterfeiting. See, for example, “Optical Interference Coatings forInhibiting Counterfeiting” by J. A. Dobrowolski, et al., Optica Acta,1973, 20, No. 12, pp. 925-937 and U.S. Pat. No. 5,009,486 byDobrowolski, et al.

Interference filters have been known for decades, as seen, for example,in U.S. Pat. No. 2,590,906 by Tripp. A typical interference filter has alargely reflective metal film on a smooth substrate. The reflective filmis overlain by a thin layer of transparent dielectric material. Thefilter is completed by a semi-reflective metal layer over thedielectric. A transparent protective coating may be applied over thereflective coating, but does not form part of the interference filteritself.

When an incident light beam encounters the front semi-reflective coatingof the interference filter, one fraction of the light is reflected andthe other fraction passes through the semi-reflective layer into thedielectric. The transmitted portion of the beam is then reflected by theback reflective layer and retransmitted through the dielectric. Afraction of the reflected wave passes through the semi-reflective frontlayer where it may constructively or destructively interfere with thereflected light.

The thickness of the dielectric material is a small multiple of aquarter wavelength of light for constructive interference (allowing forthe index of refraction of the dielectric material). Thus, when light isreflected from the interference filter, light with the appropriatewavelength has the reflected and transmitted beams in phase forconstructive interference. Light of other colors has at least partialdestructive interference. Thus, when a reflective interference filter isobserved in white light, it reflects a strong characteristic color.

The interference filter has a desirable characteristic as ananti-counterfeiting measure. The color reflected from the filter dependson the path length of light passing through the dielectric material.When the filter is observed with light at normal incidence, a certaincolor, for example blue, is seen. When the angle of incidence andreflection from the interference filter is more acute, the total pathlength through the dielectric material is longer than for normalincidence. Thus, when the interference filter is observed at an anglenearer grazing incidence, a longer wavelength color, for example purple,is observed. Such a characteristic change of color, depending on theangle of viewing the interference filter, cannot be reproduced bycopying machines.

A similar effect for transmission of a light can be obtained when theinterference filter has a thin dielectric sandwiched between twopartially reflective layers. One type of interference filter issometimes referred to as a quarter-wave plate because of itscharacteristic thickness of {fraction (1/4+L )} wavelength.

To make it even more difficult for counterfeiters, it has been proposedto use interference filter layers having different thicknesses indifferent areas. Since the color of light reflected from an interferencefilter is a function of the thickness of the dielectric material, onecan thereby achieve a multicolor effect by having different areas of thefilter with different thicknesses.

The Dobrowolski concept is to produce an interference filter using aninorganic optical coating material, such as those listed in U.S. Pat.No. 5,009,486. A layer of such material is deposited with a certainthickness. A mask is superimposed and a second layer of that material isdeposited over a portion of the first layer. Collectively, these twolayers define areas of differing thicknesses and hence, differentinterference colors.

Such a technique is costly. The metal and dielectric layers aretypically deposited on a thin film polyester substrate by a sputteringtechnique at a rate of about 3 to 10 meters per minute movement of thefilm past the deposition stations. Much faster deposition is desirable.Furthermore, two separate deposition steps with intervening masking ofthe surface must be performed to provide the two layers of dielectricwhich collectively provide a color difference.

It would be desirable to enhance the rate of formation of aninterference filter by at least an order of magnitude as compared withthe inorganic dielectric materials previously used. It is also highlydesirable to provide varying thickness of the dielectric material in theinterference filter in a single deposition step for forming a monolithiclayer of differing thickness. It is also desirable to deposit theinterference filter material in predetermined patterns of differingcolor.

BRIEF SUMMARY OF THE INVENTION

There is, therefore, provided in practice of this invention according toa presently preferred embodiment, a multicolor interference filterhaving a monolithic acrylate polymer film deposited on a substrate withsufficient thickness to produce interference color. A predetermined areaof the acrylate film has a thickness different from a second area of thefilm adjacent to the predetermined area. Thus, the predetermined areahas a different color from the adjacent area.

An interference color coating is made by evaporating an acrylate monomerhaving a molecular weight in the range of from 150 to 600 and condensingthe acrylate monomer on a substrate as a monomer film. To promoteadhesion, it is preferred that the acrylate monomer have a molecularweight to acrylate group ratio in the range of from 150 to 400. Theacrylate is polymerized for forming a film having a thickness sufficientfor producing an interference color. At least partially reflectivecoatings are provided on both faces of the polymer film. One of thecoatings may be substantially completely reflective.

A number of different techniques may be used for forming a predeterminedarea of the film with a different thickness than the adjacent area. Forexample, controlling the temperature of the substrate to be different indifferent areas controls the efficiency of deposition and, hence,thickness of the deposited film. The film shrinks as it polymerizes andone may vary the degree of polymerization in different areas to achievedifferent shrinkage and, hence, thickness. Polymerization of the filmmay be induced by electron beam or ultraviolet radiation and the degreeof polymerization is controlled by the total exposure to such radiation.Simply depositing the film with different thicknesses in adjacent areasis convenient for forming multicolored stripes, for example.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will beappreciated as the same becomes better understood by reference to thefollowing detailed description when considered in connection with theaccompanying drawings, wherein:

FIG. 1 is a fragmentary transverse schematic cross-section of areflective multicolor interference filter constructed according toprinciples of this invention;

FIG. 2 is a transverse cross-section of a multicolor interference filterwith adhesive for transfer to another substrate;

FIG. 3 is a transverse cross-section of a transmission multicolorinterference filter;

FIG. 4 is a schematic illustration of coating apparatus for forming amulticolor interference filter;

FIG. 5 is a schematic illustration of an evaporation nozzle;

FIG. 6 is a graph illustrating condensation efficiency as a function oftemperature;

FIG. 7 is a fragmentary view of the face of a drum with varying thermalproperties;

FIG. 8 illustrates a color pattern obtained from a drum as illustratedin FIG. 7;

FIG. 9 is a fragmentary view of the face of a drum with recessed areas;

FIG. 10 is a schematic illustration of additional coating apparatus;

FIG. 11 illustrates a mask for an electron gun or other radiation sourcefor polymerizing an acrylate film;

FIG. 12 illustrates a fragment of a moveable mask for a radiationsource;

FIG. 13 illustrates another variety of evaporation nozzle; and

FIG. 14 illustrates another embodiment of interference filterconstructed according to principles of this invention.

DESCRIPTION

FIG. 1 illustrates in fragmentary transverse cross-section a thin filmreflective interference filter. It will become apparent that this is aschematic illustration where the thicknesses of the various layers ofthe filter are exaggerated for clarity of illustration.

The reflective interference filter is deposited on a sheet substrate 10having a smooth surface. The substrate is typically a thin sheet ofmaterial such as polyester, polypropylene or kraft paper. Thin flexiblesubstrates are desirable for high speed coating where a roll of thesubstrate material can be continuously coated in a vacuum apparatus.Clearly, with a sacrifice in coating speed, thin films may be coatedonto rigid substrates.

The substrate is first coated with a thin metal layer 11 which islargely opaque and reflective. At least 75% reflectivity is preferable.Any easily deposited metal may be used for the reflective layer such asaluminum, chromium, nickel, nickel-chromium alloy, stainless steel,silver, or the like. Essentially colorless metals are preferred overmetals having inherent color such as copper. Typically, the opaquereflective metal layer has a thickness in the range of from 200 to 2000Angstroms.

A thin layer 12 of transparent dielectric material is deposited on thereflective layer. The thickness of the dielectric layer and its index ofrefraction determine the path length of light through the filter andhence, the color of light reflected. As can be seen at the left side ofFIG. 1, a predetermined area of the dielectric layer is formed with athickness t₁ which is different from the thickness t₂ of an adjacentarea illustrated at the right in FIG. 1. Both of these thicknesses, t₁and t₂ are in a range from about 700 to 10,000 Å and preferably in arange of from about 800 to 4000 Å. Such thicknesses are in a range forproducing an interference color from the filter. The preferreddielectric material is a monolithic acrylate polymer as describedhereinafter.

The next layer in the interference filter is a layer 13 of metal thinenough to be semi-transparent. Again, the metal layer may be any metalwhich is conveniently deposited as a thin film such as, for example,aluminum, chromium, nickel, nickel-chromium alloy, stainless steel, orsilver. Chromium is a particularly preferred material since it can bedeposited readily with a controlled reflectivity and is resistant tocorrosion. The semi-transparent (or semi-reflective) layer is at least25% transparent and is preferably about 50% transparent and 50%reflective. Typical thickness of the layer is about 50 to 200 Å.

The two metal layers may be deposited by any conventional depositiontechnique such as vacuum metalizing, or sputtering. Generally speaking,it is desirable to prepare the substrate for subsequent coatings byfirst depositing the reflective metal layer. Polyester film pre-coatedwith aluminum or other metal is commercially available and forms asuitable substrate.

After forming the interference filter with the two reflective metallayers spaced apart by a transparent dielectric, the thin films may beprotected by a deposited superstrate 14. The superstrate is a materialthat provides abrasion resistance and may have a surface that issusceptible to receipt of inks for printing over the filter. It can beeither an organic or inorganic material deposited by conventional rollercoating or vapor deposition. A preferred material is an acrylic with athickness of 2 micrometers or more.

The acrylate layer forming the dielectric material 12 of the filter ispreferably deposited in the form of vaporized acrylate monomer. Themonomer film is irradiated with ultraviolet or an electron beam to causepolymerization of the acrylate to form a monolithic film. Polymerizationby irradiation is a conventional practice and the electron flux requiredor wavelength and total flux of ultraviolet used are commonly known.

Evaporation of the monomer is preferably from flash evaporationapparatus as described in U.S. Pat. Nos. 4,722,515, 4,696,719,4,842,893, 4,954,371 and 5,097,800. These patents also describepolymerization of acrylate by radiation. In such flash evaporationapparatus, liquid acrylate monomer is injected into a heated chamber as1 to 50 micrometer droplets. The elevated temperature of the chambervaporizes the droplets to produce a monomer vapor. The monomer vaporfills a generally cylindrical chamber with a longitudinal slot forming anozzle through which the monomer vapor flows. A typical chamber behindthe nozzle is a cylinder about 10 centimeters diameter with a lengthcorresponding to the width of the substrate on which the monomer iscondensed. The walls of the chamber may be maintained at a temperaturein the order of 200 to 320° C.

Two styles of evaporator are suitable. In one of them, the orifice forinjecting droplets and flash evaporator are connected to one end of thenozzle cylinder. In the other style, the injector and flash evaporatorsection is attached in the center of the nozzle chamber like a T.

A group of acrylate resins employed from making the dielectric layer aremonomers having a molecular weight in the range of from 150 to 600.Preferably the monomers have a molecular weight in the range of from 200to 300. If the molecular weight is too low, the monomer is too volatileand does not condense well for forming a monomer film. Monomer that doesnot condense on the desired substrate may foul vacuum pumps and hinderoperation of an electron gun used for polymerizing the resin. If themolecular weight is too great, the monomer does not evaporate readily inthe flash evaporator at temperatures safely below the decompositiontemperature of the monomer.

When the monomers polymerize, there may be shrinkage of the film.Excessive shrinkage may cause poor adhesion of the film on thesubstrate. As will be explained hereinafter, some shrinkage is desirablein certain embodiments of this invention. Adhesion of the film to thesubstrate is also dependent on thickness of the film. A thin film maytolerate greater shrinkage without loss of adhesion than a thick film.Shrinkage up to about 15 to 20% can be tolerated in the thin films usedin the dielectric layer of the interference filter.

To obtain low shrinkage, there should be a relatively low crosslinkdensity. High crosslink density monomers such as hexane diol diacrylate(HDDA) and trimethylol propane diacrylate (TMPTA) have poor adhesion tometal. A way of defining crosslink density and shrinkage is to considerthe size of the molecule and the number of acrylate groups per molecule.

Preferably, the acrylate monomer has an average molecular weight toacrylate group ratio in the range of from 150 to 400. In other words, ifthe acrylate is a monoacrylate, the molecular weight is in the range offrom 150 to 400. (Actually, it is preferred that the molecular weight ofa monoacrylate be greater than 200 for other reasons.) On the otherhand, if a diacrylate is used, the molecular weight may be in the rangeof from 300 to 800. As described hereinafter, blends of acrylates ofdiffering functionality and molecular weights may also be used. In thatcase, the average molecular weight to acrylate group ratio should be inthe range of from 150 to 400. This range of values provides sufficientlylow shrinkage of the acrylate layer upon curing that good adhesion isobtained. If the molecular weight to acrylate group ratio is too high,there may be excessive shrinkage and poor adhesion to a metal layer.

Some examples of the ratio are as follows:

trimethylol propane diacrylate  98 hexane diol diacrylate 113 betacarboxy ethyl acrylate 144 tripropylene glycol diacrylate 150polyethylene glycol diacrylate 151 tripropylene glycol methyl ethermonoacrylate 260

A 50/50 blend of tripropylene glycol diacrylate and tripropylene glycolmethyl ether monoacrylate has an average ratio of 205. Higher molecularweight materials may be blended with beta carboxy ethyl acrylate (BCEA)to provide a suitable average molecular weight material.

Suitable acrylates not only have a molecular weight in the appropriaterange, they also have a “chemistry” that does not hinder adhesion.Generally, more polar acrylates have better adhesion than less polarmonomers. Long hydrocarbon chains may hinder adhesion. For example,lauryl acrylate has a long chain that is hypothesized to be aligned awayfrom the substrate and hinder polymerization, leading to relatively pooradhesion on most substrates.

A typical monomer used for flash evaporation includes an appreciableamount of diacrylate and/or triacrylate to promote polymerization.Blends of acrylates may be employed for obtaining desired evaporationand condensation characteristics and adhesion, and for controlledshrinkage of the deposited film during polymerization.

Suitable monomers are those that can be flash evaporated in a vacuumchamber at a temperature below the thermal decomposition temperature ofthe monomer and below a temperature at which polymerization occurs inless than a few seconds at the evaporation temperature. The mean time ofmonomer in the flash evaporation apparatus is typically less than onesecond. Thermal decomposition, or polymerization are to be avoided tominimize fouling of the evaporation apparatus. The monomers selectedshould also be readily capable of cross-linking when exposed toultraviolet or electron beam radiation.

The monomer composition may comprise a mixture of monoacrylates anddiacrylates. Triacrylates tend to be reactive and may polymerize at theevaporation temperatures, but may be useful in blends. Depending on thetechniques used for making a multicolored interference filter, a high orlow shrinkage film may be desirable. Generally speaking, the shrinkageis reduced with higher molecular weight materials. Blends of monomerswith different shrinkage characteristics may be employed for obtaining adesired shrinkage. Generally, for good adhesion to a metal film a lowshrinkage is desirable.

Preferably, the molecular weight of the acrylate monomer is in the rangeof from 200 to 300. If the molecular weight is less than about 200, themonomer evaporates readily, but may not condense quantitatively on thesubstrate without chilling of the substrate. If the molecular weight ismore than about 300, the monomers become increasingly difficult toevaporate and higher evaporation temperatures are required.

There are about five monoacrylates, ten diacrylates, ten to fifteentriacrylates and two or three tetraacrylates which may be included inthe composition. A particularly good acrylate is a 50:50 blend of TRPGDAand tripropylene glycol methyl ether monoacrylate with a molecularweight of about 260 (available as Henkel 8061).

Exemplayr acrylates which may be used, sometimes in combination, includemonoacrylates 2-phenoxy ethyl acrylate (M.W. 192), isobornyl acrylate(M.W. 208) and lauryl acrylate (M.W. 240), diacrylates dicetylene glycoldiacrylate (M.W. 214), neopentyl glycol diacrylate (M.W. 212) andpolyethylene glycol diacrylate (PEGDA) (M.W. 151) or tetraethyleneglycol diacrylate (M.W. 302), triacrylates trimethylol propanetriacrylate (M.W. 296) and pentaerythritol triacrylate (M.W. 298),monomethacrylates isobornyl methacrylate (M.W. 222) and 2-phenoxyethylacrylate (M.W. 206) and dimethacrylates triethylene glycoldimethacrylate (M.W. 286) and 1,6-hexanediol dimethacrylate (M.W. 254).

As has been mentioned, the nozzle for the flash evaporator typicallycomprises a slot extending longitudinally along the evaporator chamber.In an exemplary evaporator, the nozzle slot may have a width in therange of from 0.75 to 1 mm. The surface of a substrate on which themonomer is condensed may be moved past the nozzle at a distance from thenozzle of about 2 to 4 mm. Typical speed of traverse of the substratepast the nozzle is in the order of 150 to 300 meters per minute.

FIG. 2 is a fragmentary cross-section of another embodiment ofinterference filter constructed according to this invention. In thisembodiment there is a temporary substrate 16, such as a polyester filmon which there is a thin release layer 17, such as a wax or silicone. Asuperstrate material 18 is deposited on the temporary substrate over therelease layer by a technique such as flash evaporation of an acrylatemonomer. The acrylate monomer is polymerized by irradiation.

A semi-reflective metal layer 19 is then deposited on the superstrate.Thereafter, a transparent dielectric layer 21 of polymerized acrylate isformed as described above. Preferably a predetermined area of thedielectric layer has a first thickness t₁ while an adjacent area has adifferent thickness t₂. An opaque reflective metal layer 22 is depositedover the dielectric. The reflective metal layer 22, dielectric layer 21and semi-reflective metal layer 19 form an interference filter. Apermanent substrate material 23 is deposited on the opaque metal layer.Finally a layer 24 of pressure-sensitive adhesive is placed over thepermanent substrate.

This embodiment of interference filter is useful for transfer to asubstrate that is not conveniently handled in a vacuum system or forapplication to small areas of a larger substrate, such as, for example,marking limited areas on currency. When this interference filter isused, the pressure-sensitive adhesive is applied to the desiredsubstrate and the temporary substrate 16 is peeled off. This leaves thesuperstrate 18 exposed and the interference filter is then essentiallylike that described and illustrated in FIG. 1.

A part of the layers just described may be used without thepressure-sensitive adhesive to form a pigment. In such an embodiment athin protective layer of acrylate similar to the superstrate material isdeposited over a release layer and polymerized. A metal layer is thendeposited on the protective layer and a transparent layer of polymerizedacrylate is formed as described above. A second metal layer is depositedover the acrylate and a final protective layer of acrylate is added overthe metal. At least one of the metal layers is semi-reflective and bothmay be semi-reflective. The sandwich of acrylate, metal, acrylate, metaland acrylate forms a protected interference filter. This sandwich can beremoved from the release layer and broken up to form colored pigmentflakes.

FIG. 3 illustrates another embodiment of interference filter which maybe viewed from either face. A substrate 26, such as a polyester film iscoated with a thin enough layer 27 of metal that the layer issemi-reflective. A dielectric layer 28 having one thickness t₁ in apredetermined area and a different thickness t₂ in adjacent areas isformed on the semi-reflective metal. A second semi-reflective metallayer 29 is deposited over the dielectric layer and is protected by anoverlying superstrate 31 of polymerized acrylate. The twosemi-reflective layers 27 and 29, preferably each reflect about 50% andtransmit about 50% of the light incident thereon. The semi-reflectivelayers and the intervening dielectric layer form an interference filterwhich provides a multicolor appearance from either face of the sheet.

A flexible substrate material is preferable for any of these embodimentsof multicolor interference filter since a sheet of such material can berapidly coated with the multiple layers described above. A suitableapparatus for coating the substrate is illustrated schematically in FIG.4. All of the coating equipment is positioned in a conventional vacuumchamber 36. A roll of polyester sheet coated on one face with areflective layer of aluminum is mounted on a pay-out reel 38. The sheet39 is wrapped around a rotatable drum 37 and fed to a take-up reel 41.Idler rolls 42 are employed, as appropriate, for guiding the sheetmaterial from the pay-out reel to the drum and to the take-up reel.

A flash evaporator 43 as hereinabove described is mounted in proximityto the drum at a first coating station. The flash evaporator deposits alayer or film of acrylate monomer on the substrate sheet as it travelsaround the drum. After being coated with acrylate monomer the substratesheet passes an irradiation station where the acrylate is irradiated bya source 44 such as an electron gun or source of ultraviolet radiation.A movable mask 46 may be positioned on rollers for movement through thegap between the radiation source and the drum in synchronism withrotation of the drum. The radiation induces polymerization of theacrylate monomer.

The sheet then passes a metalization station 47 where a semi-reflectivecoating of metal is applied by vacuum metalizing or sputtering. Thiscompletes the interference filter. The sheet then passes another flashevaporator 48 where another layer of acrylate monomer is deposited forforming a superstrate. This layer of monomer is cured by irradiationfrom an ultraviolet or electron beam source 49 adjacent the drum.

There are a variety of techniques for producing different thicknesses ofdielectric layer in different areas of the interference filter. Broadlythere are two categories of techniques. One is to condense differentamounts of monomer in different areas of the dielectric layer to producedifferent thicknesses directly. Several techniques are suitable forthis. Alternatively, one may deposit a uniform thickness of monomer inall areas and then shrink the film thickness to different extents indifferent areas. As has been mentioned above, the acrylate monomershrinks upon polymerization. By controlling the degree of polymerizationof the film, the thickness of the dielectric layer may be controlled.Again there are different techniques for accomplishing this.

FIG. 5 illustrates schematically an arrangement for depositing differentthicknesses of acrylate monomer on different portions of a substrate. Inthis embodiment there is an elongated slot 51 forming a nozzle along thelength of a flash evaporator chamber 52. Instead of having a uniformwidth along the length of the nozzle, there are alternating narrowregions 53 and regions of wide slot 54. As will be apparent, moreacrylate is evaporated through the wide portions 54 of the nozzle thanthrough the narrower portions. As a consequence, as the substrate movespast the nozzle (perpendicular to the length of the nozzle) alternatingthick and thin stripes of acrylate monomers are deposited. Because ofthe differences in thickness in the resulting dielectric layer,alternating stripes of color such as pale green and pink are obtained.

Such a nozzle with alternating wide and narrow areas may be heldstationary as the sheet moves to produce parallel stripes.Alternatively, such a nozzle may be moved longitudinally as the sheetmoves past it and produce a zigzag pattern of colors from theinterference filter.

A somewhat analogous technique may be used for obtaining differingthicknesses in different areas of the interference filter bydifferential polymerization of a uniform thickness of monomer. Thesimilarity can be seen by reference to FIG. 11 which illustrates asource of radiation 56 such as an ultraviolet lamp or an electron beamgun. A mask 57 is interposed between the radiation source and thesubstrate with the monomer film to be polymerized (not shown in FIG.11). A slot 58 with alternating wide and narrow areas in the maskpermits passage of greater or lesser amounts of radiation. Where thegreater total flux of radiation impinges on the monomer, there is agreater degree of polymerization than in adjacent areas where the totalflux of radiation is less. Because of the inherent shrinkage of themonomer upon polymerization, the areas with greater total energy ofirradiation are thinner than the areas with less total radiation (andpolymerization). Parallel stripes of different colors are therebyobtained from the interference filter.

It should be apparent that by combining a nozzle with different widthsof slot and a radiation source with different widths of mask inappropriate alignments, three or four different colors of stripes can beobtained from an interference filter. Furthermore, the slots in masksfor deposition or polymerization may have three or more widths insteadof just two for additional color variation.

Simply having colored stripes from a multicolor interference filter maynot be considered sufficient for some purposes. The movable belt mask46, illustrated in FIG. 5 and again in FIG. 12, may be used forproducing any of a broad variety of multicolor patterns. In theschematic illustration of FIG. 12 such a movable mask has apertures 61in the form of stars and windows 62 in the form of rectangles. Just asan example, the star shaped apertures may be semi-transparent areas inan otherwise opaque belt. The rectangular windows may be completelytransparent.

Such a mask is moved in synchronism with rotation of the drum and thecoating apparatus. An ultraviolet light 41 behind the mask provides fullirradiation through the rectangular areas and partial radiation throughthe star-shaped areas. When such a mask is used it is desirable toemploy an additional source of radiation (not shown) in series with theultraviolet lamp so that all of the monomer receives at least a minimumlevel of radiation.

It will be apparent that in such an embodiment the dielectric layer willhave three different thicknesses. Most of the area irradiated by anothersource of radiation has a limited degree of polymerization and hence isrelatively thicker. The full irradiation through the open windows 62produces rectangular areas on the dielectric filter which are completelypolymerized and hence shrink to the minimum thickness. The intermediatelevel of irradiation through the star shaped openings 61, producesintermediate polymerization and intermediate thickness. The resultinginterference filter has three different colors.

A similar type of mask, having completely open windows in an otherwiseimpermeable belt, may be moved between a flash evaporator and therotating drum. The areas opposite the star and rectangular shapedopenings are coated with a layer of monomer. Another flash evaporatoreither upstream or downstream from the one occulted by the movable mask,deposits a uniform layer of monomer on the substrate. The resultingdifferent thicknesses of deposited monomer are then polymerized forproducing a monolithic acrylate film having different thicknesses indifferent areas, hence with a two dimensional multicolor pattern.

Another way of varying the thickness of acrylate monomer on thesubstrate is by controlling the efficiency of condensation. Theefficiency of condensation of the monomer is highly dependent on thetemperature of the substrate on which the monomer impinges. The effectof temperature depends on the particular monomer. An exemplaryindication of the efficiency as a function of temperature is illustratedin the graph of FIG. 6. At low temperatures such as close to 0° C.,there is essentially 100% efficiency and all of the monomer condenses.At a somewhat higher temperature, such as for example, 25° C., little,if any, of the monomer actually condenses on the substrate. It can beseen that in some temperature ranges the efficiency of condensation isquite sensitive to relatively small changes in temperature.

This temperature effect on efficiency of condensation is exploited forproducing films with monomer having different thicknesses in differentareas of the interference filter. A low temperature substrate with highefficiency of condensation produces a relatively thicker dielectriclayer whereas a somewhat higher temperature area with less efficientcondensation produces a relatively thinner dielectric layer. A number oftechniques may be used for varying the temperature of the substrate.

One way is to employ a movable mask, such as illustrated in FIG. 12immediately upstream from the flash evaporator which applies theacrylate monomer which becomes the dielectric film. An infrared lampbehind the mask irradiates areas on the substrate which absorb theinfrared radiation and are thereby heated. The higher temperature areashave less efficient condensation and hence a thinner layer of monomerthan adjacent cooler areas.

Because the efficiency of condensation changes rather steeply in thegeneral vicinity of ambient temperatures and since the flash evaporationand irradiation tend to raise the temperature of the substrate, it isdesirable to refrigerate the roll of substrate until it is placed on thepay-out reel in the coating apparatus. It is also desirable to cool therotating drum, such as for example, with chilled water, so that thesubstrate remains at a low temperature.

FIG. 7 illustrates a fragment of the surface of a. cooled drum 63 whichsupports a thin sheet substrate (not shown) as it moves through acoating apparatus. A number of ordinary paper stars 64 are glued ontothe surface of the drum. The substrate wrapped around the drum duringcoating is in good thermal contact with the drum between the stars.Where the paper stars intervene between the surface of the drum and thesubstrate, the temperature of the substrate is higher since thesubstrate is insulated from the cooling drum. As a consequence of thisthermal insulation and differential thermal pattern, there is arelatively thinner layer of monomer adjacent the stars and a relativelythicker layer elsewhere on the substrate. This results in a multicolorinterference filter.

FIG. 8 illustrates the effect achieved in an actual experiment wherepaper stars were glued on the surface of a water-cooled drum. In thearea of the star the interference filter produces a gold color withflecks of red or magenta. The color, of course, depends on the anglefrom which the interference pattern is observed. The colors mentionedare for observation at approximately normal incidence. When viewed atabout 45° the color of the star is seen as a blue-green color.Surrounding the star is a halo 66 which is a blue or blue-green color.Surrounding the halo is a band 67 of generally gold color with a reddishhue along the center of the band. The balance of the area of theinterference filter which was in better thermal contact with the drum isblue or purple in color. It is hypothesized that the band with a colorresembling that of the star may be due to a half wavelength change inthickness of the dielectric layer.

FIG. 9 illustrates another way of controlling the temperature of sheetsubstrate. FIG. 9 illustrates a fragment of the surface 68 of awater-cooled drum. The surface is covered with a pattern of shallowrecesses 69. When the thin sheet is wrapped around the drum, it is ingood thermal contact with the surface 68, but is spaced apart from thedrum surface opposite the recesses. The vacuum in the coating apparatusis an excellent insulator, and the areas over the recesses areessentially uncooled, whereas the areas in contact with the surface ofthe drum are cooled. The resulting differential temperature patternresults in differing efficiency of condensation of monomer on thesubstrate, hence differing thicknesses of dielectric layer in theinterference filter.

A very simple pattern of diagonal squares is illustrated for therecesses, but it will be apparent that any desired recess pattern may begenerated on the surface of the drum.

Such a drum is also useful in an embodiment where there is differentialpolymerization, hence differential shrinkage in a monomer. When themonomer is polymerized by an electron beam, the areas of the substratein contact with the surface 68 of the drum are well grounded. The areasopposite the recesses 69 are not as well grounded and develop anelectrostatic charge which tends to repel the electron beam. Theresulting differences in total irradiation of the electron beam on themonomer result in differences in polymerization in the dielectric layer.The resulting shrinkage differences produce multicolored images.

Electrical grounding may also be used in a different way in an electrongun, particularly for forming stripes of color along a moving sheet. Theelectron beam in a gun is accelerated by a potential difference betweenthe electron source and an accelerating screen. By varying theeffectiveness of the grounding of the screen, the electron beam can havedifferent total flux in different areas and hence produce differentialpolymerization.

It will also be apparent that an electron gun may have a narrowsteerable beam which can be used to “write” on the monomer film forproducing any desired image in multiple colors. A particularly desirableway of doing this is with an electron gun that scans across the movingsubstrate perpendicular to the direction of travel of the. substrate.The intensity of the electron beam is modulated as a function of timeduring the scan for producing variable polymerization and, in effect,scans across the film. Such a technique is quite analogous to producinga television image with a raster scan and quite intricate multicolorpatterns can be produced in an interference filter.

FIG. 10 illustrates another technique for preparing thermal imprints ona thin sheet substrate 71 before flash evaporation of an acrylatemonomer. In this coating apparatus, a refrigerated roll of substrate isplaced on a pay-out reel 72 in a vacuum system 73. The substrate passesthrough the apparatus past a number of coating and curing stations andis wound onto a take-up reel 74.

The substrate 71 first passes a metalization station 76 where one of themetal layers for forming the interference filter is deposited by vacuummetalizing or sputtering. The thin film then passes through a thermalimprinting station where the surface to receive the acrylate monomer ispassed between a backing roll 77 and a thermal imprinting roll 78. Anexemplary thermal imprinting roll has raised areas of relatively lowconductivity rubber which engaged the surface of the substrate. Therubber raised areas are contacted by a heated roll 79. The heated rollelevates the temperature of the raised areas on the imprinting roll andthe resulting thermal pattern is “printed” onto the substrate.Alternatively, the imprinting roller 78 may itself be heated or cooledfor imposing a differential temperature pattern on the substrate.

The substrate, which retains the thermal pattern temporarily in thevacuum, next passes a flash evaporation station 80 where an acrylatemonomer is condensed on the substrate. As pointed out above, thedifferential temperature results in different thicknesses of monomerbeing deposited on the substrate. The substrate then passes a radiationsource 81 where ultraviolet or an electron beam polymerizes the monomer.The film then passes the final metalization station 82 where the secondmetal layer is deposited. Additional stations may be included if desiredfor adding a protective superstrate, etc.

EXAMPLES

A multicolor interference filter can be made by evaporating acrylatemonomer through a longitudinal slot 84 as illustrated in FIG. 13 withintermittent areas 85 where the slot is essentially blocked. Such anozzle produces a coating having two colors. For example, in oneexperiment, the coating opposite an open area 84 of the slot was gold,with a bright blue line about 7 mm wide opposite the blocked area 85.The dielectric layer opposite the blocked slot was considerably thinnerthan the gold area adjacent to the blue stripe.

In another experiment, the temperature of the coating drum was changedrapidly during deposition. The drum temperature was lowered rapidly fromabout 20° C. to about 0° C. The polyester film that was in contact withthe drum experienced a temperature change which was a reflection of thechanging temperature of the drum. As the drum cooled, the color from theresultant interference filter changed from purple, to blue, to yellow.

The color patterns developed by pasting paper stars on a water cooleddrum have been illustrated in FIG. 8. In this experiment, the paperstars were either 75 or 150 micrometers thick and about 9.5 mm wide.

The variation in thickness due to differential shrinkage uponpolymerization of the acrylate was demonstrated by varying the electronflux from an electron gun. This was done by varying the groundingcondition of the accelerating screen through which the electrons pass. Agreater electron flux is emitted through areas where the screen is wellgrounded. Thus, there is a curtain of electrons with alternating lanesof heavy and light flux.

Due to the alternating high and low flux zones, the degree ofpolymerization of the acrylate differed. In these examples, thepolyester film substrate was 50 cm. wide. In one experiment three pinkstripes were interleaved with three light green stripes. Similarly,alternating blue and gold stripes were produced with a differentthickness of dielectric layer. By tripling the number of groundingpoints on the electron gun screen the number of stripes with varyingelectron density was tripled and twice as many narrower color stripesproduced.

FIG. 14 illustrates another embodiment of interference filterconstructed according to principles of this invention. In thisembodiment color is obtained by having multiple transparent layers whichare alternately materials with high index of refraction and low index ofrefraction, respectively. Interference effects due to changes in indexof refraction between the layers provide color for either a reflectiveor transparent object.

Such an object comprises a substrate 90 which may be transparent oropaque as required for a particular application. The substrate on whichthe multiple layers of acrylate are deposited may be either a rigidobject or a flexible sheet substrate. If desired, a metal layer (notshown) may be applied over the substrate for greater reflectance. Aplurality of alternating layers of low refractive index material 91 andhigh refractive index material 92 are deposited on the substrate. Aspointed out above, the thicknesses of the high and low refractive indexmaterials may be varied in different areas of the object to providedifferent colors, and in any case are appreciably less than one micron.

The acrylate employed for depositing the several layers are selected fortheir compatibility and index of refraction. Generally speaking, thefluorinated acrylates tend to have a low refractive index and aresuitable for the low index layers. An exemplary high index materialcomprises a bisphenol A diacrylate. It is preferred that the acrylatemonomer have a molecular weight to acrylate group ratio in the range offrom 150 to 400. As few as two layers may be sufficient where a hint ofcolor is sufficient. Generally, however, several layers of alternatinghigh and low index of refraction are employed.

Although a substantial number of methods for forming a multicolorinterference filter, have been described and illustrated herein, it willbe apparent to those skilled in the art that additional embodiments canreadily be devised. Other techniques may used for depositing nonuniformlayers of acrylate dielectric for producing desired color patterns.Similarly, other techniques may be used for controlling shrinkage of acondensed film of monomer by controlling the degree of polymerization.It is therefore to be understood that within the scope of the appendedclaims, this invention may be practiced otherwise than as specificallydescribed.

What is claimed is:
 1. An apparatus for forming an interference filtercomprising: at least one rotatable member capable of receiving substratematerial and of causing said substrate material to be moved; at leastone source of evaporated acrylate monomer proximate to said member; atleast one heat sink capable of inducing the condensation of saidacrylate monomer over the substrate material to form an acrylate filmhaving a first thickness and a second thickness, the first thicknessbeing greater than the second thickness, and each thickness beingsufficient for producing an interference color; and at least oneradiation source capable of polymerizing the acrylate film.
 2. Theapparatus of claim 1 wherein the evaporation source moves relative tothe substrate material.
 3. The apparatus of claim 1 further comprising amask having at least one aperture disposed between the radiation sourceand the substrate.
 4. The apparatus of claim 3 wherein the mask has morethan one aperture, the apertures being of the same dimensions.
 5. Theapparatus of claim 3 wherein the mask has more than one aperture, theapertures being of varying dimensions.
 6. The apparatus of claim 1further comprising a mask having apertures disposed between said heatsink and the substrate material.
 7. The apparatus of claim 1 wherein theevaporation source comprises an evaporation chamber and at least twonozzles coupled with the evaporation chamber, said nozzles each havingan aperture.
 8. The apparatus of claim 1 further comprising a maskhaving apertures disposed between the evaporation source and thesubstrate material.
 9. The apparatus of claim 1 further comprising atleast one metallization station depositing a metal layer over thesubstrate material.
 10. The apparatus of claim 9 further comprising oneor more metallization stations depositing a metal layer over apreviously deposited acrylate film, respectively.
 11. The apparatus ofclaim 1 further comprising at least one organic coating stationdepositing an organic layer over the substrate material, wherein one ormore organic coating stations deposit organic layers over a previouslydeposited acrylate film, respectively.
 12. The apparatus of claim 1further comprising at least one inorganic coating station depositing aninorganic layer over the substrate material, and at least one organiccoating station depositing an organic layer over the substrate material,wherein one or more inorganic coating stations deposit an inorganiclayer over a previously deposited organic layer, respectively.
 13. Theapparatus of claim 1 further comprising at least one organic coatingstation depositing at least two organic layers over the substratematerial, wherein each organic layer is composed of a different monomer.14. The apparatus of claim 1 further comprising at least one organiccoating station depositing at least two organic layers over thesubstrate material, wherein each organic layer is composed of the samemonomers.
 15. The apparatus of claim 1 further comprising at least oneinorganic coating station depositing an inorganic layer over thesubstrate material, wherein one or more inorganic coating stationsdeposit an inorganic layer over a previously deposited acrylate film,respectively.
 16. The apparatus of claim 9 further comprising at leastone inorganic coating station depositing an inorganic layer over thesubstrate material, wherein one or more metallization stations deposit ametal layer over a previously deposited inorganic layer, respectively.17. The apparatus of claim 9 further comprising at least one organiccoating station depositing an organic layer over the substrate material,wherein one or more metallization stations deposit a metal layer over apreviously deposited organic layer, respectively.
 18. The apparatus ofclaim 1 wherein said member is cooled to a temperature below ambienttemperature and material of varying dimensions is disposed between themember and substrate material.
 19. An in-line coating apparatus forforming an interference filter comprising: a substrate material providedto move in a line; at least one source of evaporated acrylate monomerproximate to said line; at least one heat sink capable of inducing thecondensation of said acrylate monomer over the substrate material toform an acrylate film; and at least one radiation source capable ofpolymerizing the acrylate film to allow the film to achieve a thicknessconsisting of one or more acrylate layers, the thickness sufficient forproducing an interference color.
 20. The in-line coating apparatus ofclaim 19 wherein the substrate material is rigid.
 21. The in-linecoating apparatus of claim 19 further comprising at least onemetallization station depositing a metal layer over the substratematerial.
 22. The in-line coating apparatus of claim 21 wherein one ormore metallization stations deposit a metal layer over a previouslydeposited acrylate layer, respectively.
 23. The in-line coatingapparatus of claim 21 further comprising at least one organic coatingstation depositing an organic layer over the substrate material, whereinone or more metallization stations deposit a metal layer over apreviously deposited organic layer, respectively.
 24. The in-linecoating apparatus of claim 19 further comprising at least one organiccoating station depositing an organic layer over the substrate material,wherein one or more organic coating stations deposit an organic layerover a previously deposited acrylate layer, respectively.
 25. Thein-line coating apparatus of claim 19 further comprising at least oneinorganic coating station depositing an inorganic layer over thesubstrate material, and at least one organic coating station depositingan organic layer over the substrate material, wherein one or moreinorganic coating stations deposit an inorganic layer over a previouslydeposited organic layer, respectively.
 26. The in-line coating apparatusof claim 21 further comprising at least one inorganic coating stationdepositing an inorganic layer over the substrate material, wherein oneor more metallization stations deposit a metal layer over a previouslydeposited inorganic layer, respectively.
 27. The in-line coatingapparatus of claim 19 further comprising at least one inorganic coatingstation depositing an inorganic layer over the substrate material,wherein one or more inorganic coating stations deposit an inorganiclayer over a previously deposited acrylate layer, respectively.
 28. Thein-line coating apparatus of claim 19 further comprising at least oneorganic coating station depositing at least two organic layers over thesubstrate material, wherein each organic layer is composed of adifferent monomer.
 29. An apparatus for forming an interference filtercomprising: at least one rotatable member capable of receiving substratematerial and of causing said substrate material to be moved; at leastone source of evaporated acrylate monomer having an evaporation chamberand at least two nozzles coupled with the at least one evaporationchamber, said nozzles each having an aperture, and at least two nozzleshaving apertures of different dimensions; at least one heat sink; atleast one radiation source; and at least one metallization stationdepositing a metal layer over the substrate material.
 30. An apparatusfor forming an interference filter comprising: at least one rotatablemember capable of receiving substrate material and of causing saidsubstrate material to be moved; at least one source of evaporatedacrylate monomer; at least one heat sink; at least one mask having atleast one aperture disposed between said at least one heat sink and thesubstrate material; and a radiation source.
 31. An apparatus for formingan interference filter comprising: at least one rotatable member capableof receiving substrate material and of causing said substrate materialto be moved, said member being cooled to a temperature below ambienttemperature and having material of varying dimensions disposed betweenthe member and substrate material; at least one source of evaporatedacrylate monomer proximate to said member; at least one heat sink; andat least one radiation source.
 32. An apparatus for forming aninterference filter comprising: at least one rotatable member capable ofreceiving substrate material and of causing said substrate material tobe moved; at least one source of evaporated acrylate monomer proximateto said member; at least one means for inducing the condensation of saidacrylate monomer over the substrate material to form an acrylate film;and at least one means of polymerizing the acrylate film to form a firstthickness and a second thickness, the first thickness being greater thanthe second thickness, and each thickness of the acrylate film beingsufficient for producing an interference color.
 33. An in-line coatingapparatus for forming an interference filter comprising: substratematerial provided to move in a line; at least one source of anevaporated acrylate monomer proximate to said line; at least one heatsink capable of inducing the condensation of said acrylate monomer overthe substrate material to form an acrylate film; at least one radiationsource capable of polymerizing the acrylate film to allow the film toachieve a thickness consisting of one or more acrylate layers, thethickness sufficient for producing an interference color; and at leastone metallization station depositing a metal layer over the substratematerial.
 34. An in-line coating apparatus for forming an interferencefilter comprising: at least one nozzle containing an evaporated acrylatemonomer and provided to move in a line; a substrate material proximateto said line; at least one heat sink capable of inducing thecondensation of said acrylate monomer over the substrate material toform an acrylate film; and at least one radiation source capable ofpolymerizing the acrylate film to allow the film to achieve a thicknessconsisting of one or more acrylate layers, the thickness sufficient forproducing an interference color.
 35. An apparatus for forming aninterference filter comprising: at least one rotatable member capable ofreceiving substrate material and of causing said substrate material tobe moved; a first reel capable of delivering the substrate material tothe rotatable member, and a second reel capable of receiving thesubstrate material from the rotatable member; at least one source ofevaporated acrylate monomer proximate to said member; at least one heatsink capable of inducing the condensation of said acrylate monomer overthe substrate material to form an acrylate film; and at least oneradiation source capable of polymerizing the acrylate film.
 36. Theapparatus of claim 35 further comprising a first idler roll positionedbetween the first reel and the rotatable member, and a second idler rollpositioned between the second reel and the rotatable member, wherein theidler rolls are capable of guiding the substrate material.